The invention relates to a visual color display tube, particularly for television, with three coplanar electron beams and of the self convergent type.
In this context, the term "tube" denotes the system formed by the tube as such, the deflector unit and the electron beam generators.
It is known that in the case of television, or more commonly stated of visual display, color pictures are normally obtained by means of a screen coated with luminescent substances having three primary colors, commonly being red, green and blue, which are excited by three electron beams, of which one is provided for each color. These beams converge on the screen and are displaced along lines extending from left to right and from top to bottom.
The screen comprises triads, that is to say groups of three dots of substances luminescent in three colors The luminous intensity (luminance) and the color of each dot of the image which is to be reproduced are obtained by means of the excitation intensity of each of the elements of the triad.
In order that each electron beam may impinge only on the dots having the color to which it is allotted, the three beams have different directions on the one hand, and a perforated mask (shadow mask) is placed in front of the screen on the other hand.
The displacement of the beams along the lines, which is referred to as scanning, is effected by means of two variable magnetic fields which deflect the beams in the horizontal direction in the case of the one, and in the vertical direction in the case of the other. These magnetic fields are generated by electrical currents of variable intensity flowing in coils referred to as deflectors.
So that the three beams may attain convergence at all points of the screen, the magnetic field traversed by these beams must have a non-uniform and particular spatial configuration which is related in particular to the rectangular and substantially plane shape of the screen and to the coplanar arrangement of the beams.
If the convergence is obtained solely by virtue of the pattern of the magnetic field, the tube is referred to as being automatic or self convergent.
This automatic convergence is primarily effected due to the form of the windings of the deflector unit. However, the structure of the coils is commonly inadequate and convergence errors normally persist. One of these faults is so-called coma, which is manifested in a tube having coplanar beams, by the dimensions of the image produced by the central beam (commonly for green) which differ from the dimensions of the image produced by the lateral beams (red and blue). This fault is measured by the distance separating two adjacent vertical edges of the gren and blue images, and by the distance separating two adjacent horizontal edges of these images.
For correction of the coma fault it is known, for example from French patent No. 2,425,146, to provide elements of high magnetic permeability behind the deflector, that is to say at the side of the electron guns, in arrangements and configurations such that the magnetic field generated by the deflector is modified in a sufficient degree to perform the correction.
The invention relates to the case in which the coma defect is of small size, that is to say of the order of one millimeter or less, and of different sense in the horizontal (east-west) and vertical (north-south) directions, the (green) image produced by the central beam being wider for example than the image produced by the lateral beams (red-blue), but of smaller height than that of this image produced by the lateral beams.
The invention derives from the discovery that the magnetic coma correction elements known until now are inappropriate for correction of a defect of this kind.
In particular, in French patent No. 2,425,146 referred to above, the magnetic elements are formed by magnetic members applied to the bottom of a cupel forming an output e1ectrode of the electron guns, and which thus has three openings for passage of the beams, two of these members having the form of crowns or rings surrounding the openings provided for the lateral beams, two other members being elongated vertical bars situated to either side of the opening for the central green beam. These magnetic elements are appropriate for coma defects of large size, of 5 mms for example, but they provide an excessive correction value for defects of lesser size thus causing a fault in the other direction.